I believe we would all agree that the formal/correct way is, 'bachelor of' or 'master of', and over time, we have discarded the 'of' for a possessive, - bachelor's or master's, for convenience.

Would it therefore be a singular possessive or a plural possessive? IMHO, it would be the plural, since the degree by its very name, draws attention to the collective profile of the persons who are obtaining such a qualification, in this case, bachelors. By such a reasoning, the usage must have shifted to bachelors', and it was but a matter of time from thereon, to drop the increasingly vestigeal looking apostrophe.

Many years ago, when learning the Korean language, I would phrase a sentence in a way which conformed with all of the rules of construction with which I was familiar. Native Korean speakers would laugh and say "We don't say it that way." When I sought yet another rule to add to my collection, they would respond "It just doesn't sound right." Immersion is the only way to acquire language proficiency beyond what is available in textbooks. This explains why small children who move to the Colonies, speaking not one word of English, acquire "better" English than do their parents, who learn the language from books.

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